Phase balance monitoring systems that respond to a phase unbalance condition and also to wrong-phase-sequence or low voltage conditions to protect a load device such as an A.C. motor have been known heretofore. However, these prior systems have had certain disadvantages among which are the following.
In prior null-balance systems wherein a null is achieved in a resistor-capacitor bridge network for a balanced set of three-phase vectors, phase unbalance is detected by the electrical signal produced upon a departure from such null condition to control a relay. In such systems, the null condition is affected not only by phase unbalance but also by frequency deviations of the line power as well as by harmonics and noise on the power line. Thus, different adjustments for a null are required for 50 or 60 Hz applications. Also, the increasing use of solid state motor drives and other power switching equipment has made the presence of electrical noise and harmonics on the power line a frequent application problem, particularly for lightly-loaded lines having power factor correction capacitors connected thereto, which capacitors can also cause tuning of the power system to a harmonic frequency. Such prior systems also typically use up to three transformers (for the bridge network and relay supply) which are bulky and require different sizes for different voltage applications. Moreover, such prior systems do not measure the true negative sequence voltage caused by the phase unbalance which would be desirable for an efficient system.
Variations of such prior null balance systems have also been known wherein the null balance is achieved in such a way that it is relatively insensitive to variations in the polyphase supply frequency and does measure the true negative sequence voltage caused by the unbalance, such as the system disclosed in J. R. Linders U.S. Pat. No. 3,699,441, dated Oct. 17, 1972. However, harmonics are still a problem unless additonal filtering is provided. Also, such systems typically require initial adjustment for 50 or 60 Hz applications and use two transformers that are bulky and must be selected for each different application voltage.
Other prior systems are of a type that develop D.C. voltages proportional to the respective phases of the monitored three-phase power line such as the system disclosed in D. R. Boothman et al U.S. Pat. No. 3,848,160, dated Nov. 12, 1974. Pairs of these voltages are then compared and the differences summed to determine the amount of phase unbalance. Such systems do not measure the true negative sequence voltage caused by the phase unbalance and besides are rather complex in that they involve a rather large amount of electronics along with transformers in each phase with their attendant disadvantages of bulkiness. Also to insure accuracy, either components with close tolerances must be used, or else, factory adjustments are required.
While these prior systems have been useful for their intended purposes, this invention relates to improvements thereover.